FIG. 1 illustrates the structure of a wireless access protocol responsible for data transmission in a radio link of a Universal Mobile Telecommunication System (UMTS) which is a third generation mobile communication system. Data link layers corresponding to the second layer (Layer 2: L2) of the Open System Interconnection (OSI) reference model include a Medium Access Control (MAC) layer, a Radio Link Control (RLC) layer, a Packet Data Convergence Protocol (PDCP) layer, and a Broadcast/Multicast Control (BMC) layer. The physical layer corresponds to the first layer (Layer 1: L1). Information exchange between protocol layers is performed through virtual access points that are referred to as “Service Access Points (SAPS)”, which are represented by ovals in FIG. 1. Data units communicated between layers are given different names. These data units are referred to as “Service Data Units (SDUs)” and basic units that protocols use for transmitting data are referred to as “Protocol Data Units (PDUs)”. In the following description of the invention, data delivered between layers in the wireless access protocol structure indicates data blocks in specific units such as SDUs or PDUs described above.
The MAC layer is a layer responsible for mapping between logical and transport channels. The MAC layer selects an appropriate transport channel for transmitting data received from the RLC layer and adds required control information to a header of a MAC PDU. Special functions performed by the MAC layer include a radio resource management function and a measurement function. The radio resource management function is not performed solely by the MAC layer. Instead, the radio resource management function serves to set operations of the MAC layer based on various MAC parameters received from a Radio Resource Control (RRC), which is located above the MAC layer, to control data transmission. Examples of the radio resource management function include a function to change mapping relations between logical and transport channels or to multiplex and transmit data through a scheduling function. The measurement function is to measure the amount of traffic of a terminal and to report the measurement to an upper layer. The upper layer can change the configuration (or setting) of the MAC layer based on the measurement information obtained by the MAC layer of the terminal, thereby efficiently managing radio (wireless) resources.
The RLC layer is located above the MAC layer and supports reliable data transmission. The RLC layer segments and concatenates RLC Service Data Units (SDUs) received from the above layer in order to construct data having a size suitable for a radio link. An RLC layer at the receiving side supports data recombination in order to restore original RLC SDUs from the received RLC PDUs. Each RLC entity can operate in a Transparent Mode (TM), an Unacknowledged Mode (UM), or an Acknowledged Mode (AM) according to processing and transmission methods of RLC SDUs. When the RLC entity operates in the TM, it transfers an RLC SDU received from an upper entity or layer to the MAC layer without adding any header information to the RLC SDU. When the RLC entity operates in the UM, it segments/concatenates RLC SDUs to construct RLC PDUs and adds header information including a sequence number to each RLC PDU. However, in the UM, the RLC entity does not support data retransmission. When the RLC entity operates in the AM, it can use the RLC SDU segmentation/concatenation function to construct RLC PDUs and can perform retransmission when packet transmission has failed. Various parameters and variables such as a transmission window, a reception window, a timer, and a counter are used for the retransmission function in the AM.
The PDCP layer is used only in packet exchange regions and can compress and transmit IP packet headers so as to increase the transmission efficiency of packet data in wireless channels. The PDCP layer also manages sequence numbers in order to prevent data loss during Serving RNC (SRNC) relocation.
The BMC layer broadcasts cell broadcast messages received from a core network to multiple users through a common channel.
The physical layer, which is the first layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to the Media Access Control (MAC) layer located above the physical layer through a transport channel. Data is transferred between the MAC layer and the physical layer through the transport channel. Data is transferred between different physical layers (specifically, physical layers of transmitting and receiving sides) through a physical channel.
A Radio Resource Control (RRC) layer, which is the third layer located at the bottom, is defined only in the control plane and is responsible for controlling logical, transport, and physical channels in association with configuration, re-configuration, and release of Radio Bearers (RBs). RBs are services that the second layer provides for data communication between terminals and a network including a base station. The control plane is a hierarchy in which control information is transferred in the vertical structure of the wireless access protocol of FIG. 1 and the user plane is a hierarchy in which user information such as data/information is transferred.
As shown in FIG. 1, an RLC PDU generated in the RLC layer is transferred to the MAC layer and is handled as a MAC SDU in the MAC layer. While a MAC SDU, which is an RLC PDU received from the RLC layer, undergoes various functions of the MAC layer, various header information required for data processing is added to the MAC SDU. The header information can be altered depending on mapping relations between logical and transport channels.
Logical channels provide transport passages required for data exchange between the MAC and the RLC layer. Each logical channel is classified into control and traffic channels according to the type of data transmitted therethrough. The control channel transmits data of the control plane and the traffic channel transmits user traffic data. A logical channel is a type of data stream carrying a specific type of information. Each logical channel is generally connected to one RLC entity. One or more logical channels of the same type can also be connected to an RLC entity. Transport channels provide passages for data communication between the physical and MAC layers. A data stream in a logical channel is embodied as a MAC PDU in the MAC layer. Reference will now be made to the MAC PDU.
FIG. 2 illustrates an example structure of a MAC PDU with a header added thereto in a mobile communication system. A MAC PDU includes one or more MAC SDUs corresponding to payload for data and a MAC header which is a set of MAC sub-headers indicating the size or type of each MAC SDU. In the example of FIG. 2, it is assumed that a total of N upper layer data blocks are provided. A MAC sub-header includes a Logical Channel ID (LCID) identifying each SDU, a length field (L) indicating the size of each SDU, and an extension field (E) indicating whether a subsequent field is a MAC header or an SDU to indicate whether or not additional headers are present.
The LCID indicates which logical channel corresponds to data of a MAC SDU which is an upper layer data block associated with a sub-header including the LCID. That is, one MAC PDU includes one or more upper layer data blocks and different logical channels can be allocated to the upper layer data blocks individually.
Generally, one or more logical channels can be established between a terminal and a base station. For example, in the case of a voice service, not only a logical channel carrying voice traffic but also a logical channel for a Signaling Radio Bearer (SRB) for control information communicated between the base station and the terminal can be established between the base station and the terminal. In this case, state changes of the SRB and state changes of the voice traffic channel may occur independently of each other. More specifically, change of an Adaptive Multi-Rate (AMR) codec mode in the voice traffic channel and generation of an urgent message in the SRB channel may occur independently of each other. Therefore, an upper layer data block for voice traffic and an upper layer data block for an SRB can both be allocated to one MAC PDU and the size or the like of each upper layer data block can be set to be different according to the type and usage of a logical channel associated with the upper layer data block. The size of each upper layer data block can be set through a size field in a MAC sub-field of a MAC SDU corresponding to the upper layer data block.